profileAAR – A QGIS Plugin for preparing to rectify archaeological section images
01.10.2021
Summary
With the workflow presented here with the aid of the QGIS plugins profileAAR and georeferencer, it can be shown that a comparable result can be achieved for rectifying archaeological profile photos with the open source applications as with the proprietary software that was previously in use. Disadvantages compared to the CAD-based way become apparent when the profiles are to be viewed and processed in 3D space. The differences in the compared measuring sections are deviations of less than one centimeter for 29 profiles, which is to be regarded as sufficient for the application. The method can thus be seen as an alternative, at least for simple applications.
Statement of need / Introduction
For a long time, the standard method for documenting archaeological profiles and plana has been drawing in the field. In addition to the classic hand drawings, new, faster innovations came up in this area. This certainly included the pantograph in which an analog drawing was created or the Trigomat, which enabled direct digital drawing. With the advent of total station measurement and the connection to CAD systems, more and more recording was shifted directly to the PC. In order to include photographs of planes and profiles in these measurements, image-based methods (e.g. structure from motion) are of course available for more complex questions, but photogrammetry is also a possibility to integrate profiles and planes. In order to implement this on the software side, the AutoCAD plugin PhoToPlan (Formerly from Kubit, now Faro AsBuild Photo) has prevailed (Gütter 2015). It should be noted, however, that both AutoCAD and the plug-in are licensed and expensive to use. In the context of the increasing focus on the use of open source GIS in archeology and also in excavation technology (e.g. survey2gis or tachy2gis), the aim was to evaluate to what extent the functionality of the rectification of profiles and plans in QGIS was possible to implement. Using the georeferencer plug-in integrated in QGIS, it was previously possible to display Plana, but this was more difficult with profiles. The problem here is that the points measured in the 3D space map the profile, but QGIS can only work with the 2.5D space. The plug-in profileAAR (Mennenga, Schmütz, and Rinne 2019) was developed to transform the points in such a way that rectification in QGIS is possible. It is possible to document profiles using SFM and thus form the basis for drawings and the like. In many cases (simple post holes or sectional profiles) the effort involved is not in proportion to the result, so that the possibility of rectifying single images should still be available. In the following, the functionality of the plug-in and the rectification of profiles, but also the accuracy in comparison with the rectification in the previous approach using Faro AsBuild are presented.
profileAAR
Fig. 1: profileAAR Plugin.
The aim of the plug-in profileAAR (Fig. 1) was to create the possibility to transform the measured photogrammetry points in such a way that it is possible to rectify the photographs using the georeferencer integrated in QGIS. The Plugin is available on github and in the qgis extension manager (QGIS Plugin Repository 2021). To do this, they had to be moved from their position to the 2D view in such a way that they could be used. In a classic hand drawing, two nails are attached to the profile at the same height and these are connected with a cord that is used as a zero line. This line is measured vertically up and down as possible in order to obtain the support points for a drawing. This means that the profile, whether straight or crooked, is in principle projected onto a vertical plane. Theoretically, I can achieve the same thing with photography if I align the camera horizontally in a balance and vertically orthogonally on the profile. Since this is hardly feasible in practice, georeferencing is necessary. As a basis, a file is required in which the points are in a Cartesian coordinate system and the following parameters are available in columns at each point: z-value, profile number, view of the profile and whether the points should be used for calculating the rotation parameters (Tab. 1).
| z-Value | section_no | use |
|---|---|---|
| 0.5 | 1 | N |
| 0.5 | 1 | N |
| 0.25 | 1 | N |
| 0 | 1 | N |
| 0 | 1 | N |
| 1.8 | 2 | E |
| 1.5 | 2 | E |
| 1 | 2 | E |
| 0.75 | 2 | E |
| 0.8 | 2 | E |
Tab. 1: Example of an attribute table
Several steps are necessary to prepare the profile nails for use - the points must be rotated around the z-axis so that the profile is parallel to the x-axis. In a subsequent step, they must then be rotated around the X axis. For each profile it must first be determined how it can be aligned parallel to the x-axis. In order to determine the rotation parameters, the mean in the point cloud of a profile is determined using a linear regression (Backhaus et al. 2011) and the angle of rotation can then be calculated on the basis of the gradient. There are two important parameters that can be specified. On the one hand, it is important to know from which direction the photo was taken (view Fig. 1) and which points should be used to determine the rotation angle (used Fig. 1). With the latter, outliers in the measurement can be removed for the parameter determination - but they are still rotated (this procedure also allows finding points or similar to be attached to the profile, but only to rotate them and thus represent them on the profile). After determining the angle, the points are now aligned parallel to the X axis via translation and rotation. (In this context, the Direction selection menu (Fig. 1) can be used to determine whether the points are parallel to the profile section (original), parallel to the x-axis (horizontal) or parallel to the x-axis but with the smallest x-value = 0 ( original heigth). The latter enables a coordinate frame to be displayed around the profile in the layout function of Qgis.) In the next step, the profile is folded in such a way that it is no longer vertical in space, but lies on the 2D surface. In the simple case, the z and y coordinates are swapped (Fig. 1 Method - Projected). This method methodically produces the result that is also achieved when a classic drawing is carried out in the field, as described above (Fig. 2). It is also possible to carry out this step in such a way that an orthogonal view of the profile is generated (Fig. 1 Method - Surface; Fig. 2-2). The procedure is the same as for the rotation around the z-axis.
Fig. 2: Schematic representation of the various options in the method menu.
To complete the functions, there is still the option of extracting the upper right photogrammetry points in order to be able to use them in the layout as height markers and the profile section, based on the outermost, highest nails, can be extracted as a shape file. The georeferencer can now be used to finally rectify the photos of the profiles. After loading the images, the photogrammetry nails can now be picked up and connected to the rotated nails from profileAAR. In the following, the image is rectified using projective transformation (Documentation 2021).
Method for compairing the procedures
In order to compare the results of the rectification in QGIS with the previous and common method in AutoCAD, a test series of 29 profiles was rectified in both QGIS and AutoCAD. Since the processing of the images in the programs is very different and a pixel-by-pixel comparison cannot be carried out, a manual approach was therefore chosen. In order to be able to compare the procedures, the procedure had to be adapted as detailed as possible. For this purpose, a free UCS was first placed over the measured photogrammetry points in AsBuild Photo and the image on it was rectified. In the next step, a vertical UCS was created over the points and several points that were distributed and easily identified were marked in the image. The same points were then also marked in the rectified image in the GIS and the distances between the points were measured (Fig. 3). These distances form the basis of the comparison (Tab. 2).
| Profile no. | Axis no. | Distance [cm] Qgis | Distance [cm] CAD | Area | No. of pictures | Difference [cm] |
|---|---|---|---|---|---|---|
| 16 | 1 | 85.663 | 85.779 | 0.2 m² | 1 | -0.116 |
| 16 | 2 | 38.643 | 38.696 | 1.4 x 0.23 m | -0.053 | |
| 16 | 3 | 32.796 | 32.788 | 0.008 | ||
| 16 | 4 | 31.372 | 31.324 | 0.048 | ||
| 23 | 1 | 32.326 | 32.348 | 0.5 m² | 1 | -0.022 |
| 23 | 2 | 80.669 | 80.64 | 2.35 x 0.55m | 0.029 | |
| 23 | 3 | 153.927 | 153.929 | -0.002 | ||
| 23 | 4 | 21.202 | 21.207 | -0.005 | ||
| 24 | 1 | 52.858 | 52.814 | 0.25 m² | 1 | 0.044 |
| 24 | 2 | 107.371 | 107.257 | 0.3 x 1.3m | 0.114 | |
| 24 | 3 | 26.219 | 26.222 | -0.003 | ||
| 24 | 4 | 35.246 | 35.173 | 0.073 | ||
| 31 | 1 | 64.323 | 64.155 | 1.2 m² | 1 | 0.168 |
| 31 | 2 | 111.203 | 111.178 | 1.15 x 1.3m | 0.025 | |
| 31 | 3 | 82.778 | 82.68 | 0.098 | ||
| 31 | 4 | 83.515 | 83.403 | 0.112 | ||
| 47 | 3 | 524.257 | 524.033 | 8.7m² | 1 | 0.224 |
| 47 | 1 | 497.226 | 496.942 | 7 x 1.4m | 0.284 | |
| 47 | 2 | 128.589 | 128.611 | -0.022 | ||
| 47 | 4 | 199.206 | 198.928 | 0.278 | ||
| 52 | 1 | 226.563 | 226.716 | 6 m² | 1 | -0.153 |
| 52 | 3 | 163.322 | 163.198 | 2.15 x 3.5m | 0.124 | |
| 52 | 4 | 181.653 | 181.874 | -0.221 | ||
| 52 | 2 | 174.268 | 174.274 | -0.006 | ||
| 147 | 4 | 565.224 | 565.044 | 6.5 m² | 3 | 0.18 |
| 147 | 3 | 382.059 | 381.772 | 7 x 1.2m | 0.287 | |
| 147 | 2 | 137.639 | 137.472 | 0.167 | ||
| 147 | 1 | 365.551 | 365.506 | 0.045 | ||
| 536 | 4 | 98.998 | 99.111 | 11 m² | 3 | -0.113 |
| 536 | 3 | 836.468 | 836.576 | 10 x 1.1m | -0.108 | |
| 536 | 2 | 277.151 | 277.219 | -0.068 | ||
| 536 | 1 | 345.192 | 345.244 | -0.052 | ||
| 321 | 1 | 138.29 | 138.323 | 1 m² | 1 | -0.033 |
| 321 | 2 | 40.361 | 40.403 | 1.6 x 0,7 m | -0.042 | |
| 321 | 3 | 105.439 | 105.491 | -0.052 | ||
| 321 | 4 | 83.741 | 83.761 | -0.02 | ||
| 334 | 1 | 129.633 | 129.641 | 1.2m² | 1 | -0.008 |
| 334 | 2 | 60.907 | 60.932 | 1.6 x 0.8m | -0.025 | |
| 334 | 3 | 108.209 | 108.27 | -0.061 | ||
| 334 | 4 | 80.202 | 80.215 | -0.013 | ||
| 412 | 1 | 190.447 | 190.397 | 1.3m² | 1 | 0.05 |
| 412 | 2 | 40.139 | 40.045 | 2.6 x 0.5m | 0.094 | |
| 412 | 3 | 137.58 | 137.604 | -0.024 | ||
| 412 | 4 | 166.303 | 166.303 | 0 | ||
| 398 | 1 | 70.943 | 71.04 | 0.6m² | 1 | -0.097 |
| 398 | 2 | 58.798 | 58.751 | 0.9 x 0.8m | 0.047 | |
| 398 | 3 | 76.393 | 76.56 | -0.167 | ||
| 398 | 4 | 48.634 | 48.576 | 0.058 | ||
| 554 | 1 | 115.586 | 115.568 | 1.2m² | 1 | 0.018 |
| 554 | 2 | 65 | 65.012 | 1.2 x 1.3m | -0.012 | |
| 554 | 3 | 58.074 | 58.038 | 0.036 | ||
| 554 | 4 | 89.594 | 89.632 | -0.038 | ||
| 10 | 1 | 139.878 | 139.69 | 1.66m² | 1 | 0.188 |
| 10 | 2 | 59.091 | 58.918 | 0.9 x 1.8m | 0.173 | |
| 10 | 3 | 101.785 | 101.618 | 0.167 | ||
| 10 | 4 | 90.462 | 90.398 | 0.064 | ||
| 13 | 1 | 172.519 | 172.207 | 2.9m² | 1 | 0.312 |
| 13 | 2 | 65.837 | 65.798 | 0.9 x2.29m | 0.039 | |
| 13 | 3 | 160.1 | 159.987 | 0.113 | ||
| 13 | 4 | 108.682 | 108.352 | 0.33 | ||
| 51 | 1 | 193.345 | 193.644 | 4.34m² | 1 | -0.299 |
| 51 | 2 | 187.316 | 187.082 | 2.1 x 2.4m | 0.234 | |
| 51 | 3 | 139.612 | 139.719 | -0.107 | ||
| 51 | 4 | 89.815 | 90.003 | -0.188 | ||
| 97 | 1 | 283.48 | 283.324 | 3.92m² | 1 | 0.156 |
| 97 | 2 | 101.208 | 101.284 | 1.5 x 3.34m | -0.076 | |
| 97 | 3 | 156.24 | 156.252 | -0.012 | ||
| 97 | 4 | 212.833 | 212.17 | 0.663 | ||
| 176 | 1 | 201.638 | 201.939 | 3.25m² | 1 | -0.301 |
| 176 | 2 | 101.637 | 101.564 | 1.34 x 2.4m | 0.073 | |
| 176 | 3 | 137.343 | 137.504 | -0.161 | ||
| 176 | 4 | 72.319 | 72.273 | 0.046 | ||
| 47/95 | 1 | 641.496 | 641.418 | 8.9m² | 2 | 0.078 |
| 47/95 | 2 | 105.265 | 105.423 | 6.98 x 1.28m | -0.158 | |
| 47/95 | 3 | 314.418 | 314.436 | -0.018 | ||
| 47/95 | 4 | 379.341 | 379.324 | 0.017 | ||
| 147 | 1 | 627.263 | 627.529 | 6.37m² | 3 | -0.266 |
| 147 | 2 | 94.734 | 94.746 | 7 x 1.2m | -0.012 | |
| 147 | 3 | 296.632 | 296.837 | -0.205 | ||
| 147 | 4 | 280.471 | 280.329 | 0.142 | ||
| 247 | 1 | 212.186 | 212.066 | 4.08m² | 2 | 0.12 |
| 247 | 2 | 205.946 | 205.589 | 2.5 x 2.4m | 0.357 | |
| 247 | 3 | 197.994 | 198.416 | -0.422 | ||
| 247 | 4 | 159.828 | 159.587 | 0.241 | ||
| 246 | 1 | 250.659 | 251.987 | 4.8m² | 2 | -1.328 |
| 246 | 2 | 208.503 | 208.436 | 3.1 x 2.2m | 0.067 | |
| 246 | 3 | 262.904 | 263.441 | -0.537 | ||
| 246 | 4 | 158.483 | 159.26 | -0.777 | ||
| 6 | 1 | 47.69 | 48.276 | 0.75 m² | 1 | -0.586 |
| 6 | 2 | 106.638 | 106.873 | 1.3 x 0.65 m | -0.235 | |
| 6 | 3 | 97.007 | 97.63 | -0.623 | ||
| 6 | 4 | 92.938 | 92.535 | 0.403 | ||
| 12 | 1 | 117.916 | 117.911 | 0.668m² | 1 | 0.005 |
| 12 | 2 | 26.988 | 26.698 | 0.46 x 1.5m | 0.29 | |
| 12 | 3 | 79.628 | 79.695 | -0.067 | ||
| 12 | 4 | 68.86 | 68.566 | 0.294 | ||
| 11 | 1 | 138.311 | 138.297 | 0.712m² | 1 | 0.014 |
| 11 | 2 | 45.143 | 45.175 | 0.54 x 1.75m | -0.032 | |
| 11 | 3 | 118.281 | 118.245 | 0.036 | ||
| 11 | 4 | 78.148 | 78.119 | 0.029 | ||
| 13 | 1 | 41.371 | 41.386 | 0.424m² | 1 | -0.015 |
| 13 | 2 | 39.893 | 39.872 | 0.47 x 0.52m | 0.021 | |
| 13 | 3 | 38.962 | 38.811 | 0.151 | ||
| 13 | 4 | 31.561 | 31.72 | -0.159 | ||
| 15 | 1 | 122.66 | 122.627 | 0.831m² | 1 | 0.033 |
| 15 | 2 | 46.402 | 46.337 | 1.27 x 0.68m | 0.065 | |
| 15 | 3 | 83.043 | 83.069 | -0.026 | ||
| 15 | 4 | 63.647 | 63.633 | 0.014 | ||
| 20 | 1 | 132.39 | 132.401 | 0.798m² | 1 | -0.011 |
| 20 | 2 | 41.085 | 41.138 | 0.6 x 1.41m | -0.053 | |
| 20 | 3 | 93.914 | 94.003 | -0.089 | ||
| 20 | 4 | 65.114 | 64.389 | 0.725 | ||
| 32 | 1 | 134.232 | 134.13 | 1.98m² | 1 | 0.102 |
| 32 | 2 | 92.02 | 91.977 | 1.16 x 1.72m | 0.043 | |
| 32 | 3 | 123.213 | 123.185 | 0.028 | ||
| 32 | 4 | 102.014 | 101.938 | 0.076 | ||
| 5 | 1 | 91.013 | 90.945 | 0.391m² | 1 | 0.068 |
| 5 | 2 | 36.363 | 35.739 | 1.08 x 0.39m | 0.624 | |
| 5 | 3 | 57.404 | 57.165 | 0.239 | ||
| 5 | 4 | 57.122 | 56.96 | 0.162 | ||
| 8 | 1 | 27.56 | 27.56 | 0.626 m² | 1 | 0 |
| 8 | 3 | 44.699 | 44.627 | 0.43 x 1.18m | 0.072 | |
| 8 | 4 | 91.483 | 91.209 | 0.274 | ||
| 8 | 2 | 79.724 | 79.71 | 0.014 |